In the telephone system currently in use today, which conveys analog voice and digital data communications, it is necessary to convey a number of channels, each carrying a discrete voice or data communication, over a single twisted pair of conductors or a fiber optic cable. In order to accomplish this, the channels are multiplexed at a frequency depending on the number of channels applied to the pair of conductors. For example, in the instance where a PBX (private branch exchange) is installed in an office building, analog voice communications for each channel are coupled to a PBX switching and multiplexing unit in the building. Here, the signals are digitized by an A/D converter and multiplexed by time division multiplexing, and output to the twisted pair of conductors leading away from the office building as a serial, bipolar data stream at a frequency of about 1.544 Mhz. At this frequency, up to 24 channels may be conveyed on the single pair of conductors, with this frequency and numbers designated as a T1 signal. This T1 signal is routed to a switching station outside the building or to a central office proximate the destination where the discrete 24 channels are demultiplexed and directed to their destination as an analog signal. In the instance where the call is a long distance call, connections are made to an area network, which includes a multiplexer that multiplexes the various channels at a frequency of about 44.736 Mhz, and which will carry the equivalent of 28 multiplexed T1 channels, or 672 discrete channels, over a single pair of conductors. This frequency and number of channels is designated as a T3 signal. This scheme of multiplexing discrete channels at higher frequencies continues to very high data rates which are suitable for overseas communications between continents over fiber optic conductors, and which operates in the gigahertz range to convey over 25,000 channels over a single fiber optic light guide.
The multiplexed T1 signal is applied to the twisted pair transmission line as a wideband bipolar signal having positive and negative pulses, and having a maximum voltage of about 6 volts peak-to-peak, or about 3 volts for each of the positive going and negative going pulses. Frequency content of the signal is over a wide band of frequencies from approximately 500 Khz to greater than 4.5 Mhz, with attenuation from the signal line, which has a characteristic impedance of about 100 ohms, causing attenuation of the signal at a rate of about 20 dB per mile at the Nyquist frequency (one half the bit rate) of approximately 772 Khz, with significantly greater attenuation of the higher frequency signal components and significantly less attenuation of the lower frequency signal components. This necessitates the use of repeater circuitry to regenerate the signal at intervals of about every 6000 to 9000 feet of the twisted pair line, which repeater circuitry serving to receive a nominal signal of about 0.6 volts peak to peak, perform a spectral equalization of the signal, and output the signal at about 6 volts peak to peak.
The equalization function consists of a filter which has an inverse attenuation versus frequency transfer function of the twisted pair transmission line; for example, providing 20 dB gain at the 772 Khz Nyquist frequency, greater gain at higher frequencies, and less gain at lower frequencies, resulting in an "equalized", or flat, broadband frequency transfer function through a 6000 foot cable section and equalizing amplifier section of the repeater circuitry. This equalized signal consists of a band limited (for noise reduction) bipolar pulse sequence, the pulses having a uniform amplitude and undistorted sinusoidal wave shape for optimum data detection and clock signal extraction. When a receiver equalizer waveform is observed with an oscilloscope with multiple traces of different pseudorandom data sequences overlayed, there appears a distinctive "eye" pattern which is indicative of a properly designed equalizer operating at optimum fidelity with an unimpaired transmission line to produce maximum signal margins for the receiver data detector. These margins are disposed both above and below voltage thresholds at positive and negative 650 millivolts and positive and negative 700 millivolts dividing ZERO and ONE logic levels, respectively, as shown in FIG. 1. In contrast, FIG. 1a shows "eye" pattern distortion wherein ONE logic levels are degraded to a point where they are less than nominal amplitude but extend above or below the positive and negative 700 millivolt thresholds, as indicated by the "SHORT ONE" designation. Likewise, ZERO logic levels which are below the 650 millivolt thresholds but are greater than nominal ZERO amplitudes are indicated by the "TALL ZERO" designation.
Generally, this system works well, but there are a number of factors that can cause the signal to be degraded to a point where receiving circuitry may not be able to recover the bipolar signals, or may only be able to recover a portion of the signals. For instance, when water leaks into a cable housing a number of twisted pair communication lines, the signal is attenuated to a greater extent and may be degraded to an unusable level. In the instance where one of the twisted pairs is broken and the two broken ends are touching, or a grounded conductor is just touching one or both of the twisted pair conductors, the signal may be intermittently propagated depending on thermal expansion or contraction of the cable. In this instance, the signal may only marginally possess sufficient energy to be regenerated. Additionally, other signal lines inadvertently spliced in parallel with a T1 span cause disruptions to the signal by causing signal reflection and impedance mismatches. Further, induced current into the signal lines may cause an offset in D.C. potential of the signal, causing the T1 signal to ride on the offset D.C. potential. In instances of the aforementioned problems, and with other physical problems with the signal line, distortion of the transmitted signal is caused. This distortion may severely degrade margins of the received signals, but only cause detectable receiver data errors intermittently depending on the magnitude and frequency of interfering noise sources. An impaired circuit may show no errors in the absence of interfering noise, while the same impaired circuit displays detectable errors in a high noise environment. In these instances where noise causes impairment in the signal line, fault isolation generally requires taking the line out of service, and performing a Bit Error Rate Test (BERT) and Time Domain Reflectometer (TDR) test to detect and isolate the impaired span of transmission line. This results in significant "down time" during which customer service is interrupted. Also, many marginal conditions are never detected until changing noise environments degrade margins of the T1 signal to levels that have deleterious effects on the data carried thereby.
In accordance with the foregoing, it is an object of this invention to provide a method and system for detecting degree of impairment of a signal line without taking the line out of service, and for displaying such impairment in terms of a percentile of line impairment.